Drilling operations for the recovery of offshore deposits of crude oil and natural gas are taking place in deeper and deeper waters. Drilling operations in deeper waters are typically carried out from floating vessels rather than from stationary platforms resting on the ocean floor and commonly used in shallow water. According to conventional procedures, a drilling vessel is dynamically stationed, or moored, above a well site on the ocean floor. After a wellhead has been established, a blowout preventer (BOP) stack is mounted on the wellhead to control the pressure at the surface.
Subsea well boreholes are typically drilled with multiple sections having decreasing diameters as the wellbore extends deeper into the earth. Each borehole is cased with a casing string that extends into the borehole from a wellhead and is cemented within the borehole. The drilling, casing installation, and cementing are performed through one or more risers that extend from the wellhead to the surface, such as to a floating drilling vessel.
A riser pipe extends from the floating vessel to the wellhead equipment on the ocean floor to conduct downhole operations. The riser is attached to the wellhead equipment and is supported in tension at or near the water surface. In drilling the borehole for the well, a drill string is passed from the floating vessel down through the riser and wellhead equipment and into the borehole.
The floating drilling vessel applies tension at the top of the riser to support the weight of the riser and the drilling fluid in the riser. This necessitates that the riser have sufficient strength to handle the tension thereby requiring that the thickness of the wall of the riser be increased which in turn increases the weight of the riser. The more weight that is required, the greater the tension that is required.
Drilling mud is circulated down through the drill string and returned to the vessel through the annulus formed between the riser and the drill pipe. It is necessary for the riser, extending several thousand feet, to handle the pressure of all of the drilling mud needed for drilling the borehole sections. The difference in density between the drilling mud and sea water causes the fluid column in the riser to create a large pressure differential that must be contained within the riser. The column of drilling mud can be approximately twice as heavy as sea water such that for every foot of depth, there is about one-half psi of mud gradient weight so that at a depth of 10,000 feet, there could be 5,000 psi inside the riser relative to the sea water around the riser.
The drilling fluids in the riser also form a fluid column placing a hydrostatic head on the well for well control purposes. Well control is established by maintaining the density of the drilling fluid, and thus the hydrostatic pressure exerted on the subsurface formations, at a level that is sufficient to prevent the production fluids under pressure in the formation from overcoming the hydrostatic head. If the hydrostatic head on the well is insufficient, the pressurized gas and other formation fluids may exceed the hydrostatic head leading to a blowout.
On the other hand, if the hydrostatic head is too great, the hydrostatic head may force drilling fluids into the formation causing the loss of drilling fluids into the formation or a reduction or loss in production. If too much drilling fluid is lost into the formation and the level of drilling fluid drops in the riser, the hydrostatic head can decrease below the pressure of the formation and cause a blowout. Furthermore, the hydrostatic head may increase to an amount so as to fracture the formation resulting in increased lost circulation.
According to conventional practice, various auxiliary fluid lines may be coupled to the exterior of the riser tube. Exemplary auxiliary fluid lines include choke, kill, booster, glycol, and hydraulic fluid lines. Choke and kill lines typically extend from the drilling vessel to the wellhead to provide fluid communication for well control and circulation. The choke line is in fluid communication with the borehole at the wellhead and bypasses the riser to vent gases or other formation fluids directly to the surface. A surface-mounted choke valve is connected to the terminal end of the choke conduit line. The downhole back pressure can be maintained substantially in equilibrium with the hydrostatic pressure of the column of drilling fluid in the riser annulus by adjusting the discharge rate through the choke valve.
The kill line is primarily used to control the density of the drilling mud. One method of controlling the density of the drilling mud is by the injection of relatively lighter drilling fluid through the kill line into the bottom of the riser to decrease the density of the drilling mud in the riser. On the other hand, if it is desired to increase mud density, a heavier drilling mud is injected through the kill line.
The booster line allows additional mud to be pumped to a desired location so as to increase fluid velocity above that point and thereby improve the conveyance of drill cuttings to the surface. The booster line can also be used to modify the density of the mud in the annulus. By pumping lighter or heavier mud through the booster line, the average mud density above the booster connection point can be varied. While the auxiliary lines provide pressure control means to supplement the hydrostatic control resulting from the fluid column in the riser, the riser tube itself provides the primary fluid conduit to the surface.
In some riser systems the auxiliary fluid lines cooperate with the riser main tube to share the tensile forces applied to support the riser.